1. Field of the Invention
This invention relates to loose source routing methods that perform routing of IP packets (where "IP" is an abbreviation for "Internet Protocol"), which are transferred through ATM networks (where "ATM" is an abbreviation for "Asynchronous Transfer Mode"). Particularly, this invention relates to the loose source routing method in which a transmission source gateway basically designates a passing node of the ATM network through which ATM cells corresponding to the IP packet pass. This application is based on patent application No. Hei 9-202591 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
Recent tendency of telecommunications increase demands to provide application of local connectionless characteristics of user LANs (an abbreviation for "Local Area Network") to the global ATM networks with high speed and low cost.
For instance, the paper of Japanese Patent Application, Publication No. 4-138739 discloses an example of the routing method of ATM cells, which is achieved as follows:
At the routing of connectionless packets such as IP packets over the ATM networks, ATM cells are placed on virtual connections (abbreviated by "VC") corresponding to a direction of a destination user interface at the passing node of the ATM network. Herein, the destination user interface is written on a top cell of the packet, which is called a BOM cell (where "BOM" is an abbreviation for "Beginning Of Message").
FIG. 9 is an algorithm diagram showing a conventional example of the routing method of connectionless packets in the ATM network. Herein, a series of received ATM cells pass through an ATM node. In the case of the BOM cell within the series of ATM cells, the routing of the BOM cell is performed with reference to destination address E.164 stored in the BOM cell. As for the cells other than the BOM cell within the series of ATM cells, the routing is performed with reference to the content of the aforementioned routing of the BOM cell.
Conventionally, the source routing method, which is designed such that a transfer path is designated by a transmission source for transfer of an IP packet, is used to designate the transfer path in response to the policy of the transmission source and is used to configure a specific service network on the existing network.
The IP packet communication, which is represented by "Ipv4" (i.e., IP Version 4), for example, defines a technique called "tunneling" as the source routing method for designation of the transfer path. FIG. 10 is a conceptual diagram showing the source routing method employing the "tunneling" of the IP packets.
According to this method, an IP packet to be transmitted through a designated path is capsulated by an IP header whose destination address corresponds to an address of a passing node at first. Then, the capsulated IP packet is transmitted toward the passing node. Within a transmission path terminated by the passing node, transmission of the packet is performed using the destination address of the IP packet used for capsulation of the packet. At the passing node which receives the packet addressed thereto, the passing node dissolves the capsulation of the packet. So, the IP packet is transferred to a real address which corresponds to the original destination address of the IP packet.
In addition, the conventional technology provides combinations of the aforementioned techniques for the source routing of the IP packets on the ATM network. That is, the conventional technology provides a method that at a passing node, cells are assembled into an IP packet, which is relayed to another node. For example, the paper of Japanese Patent Application, Publication No. 8-56227 discloses the technology that an ATM communication node is equipped with a packet relay unit, which has a frame assembling section for assembling ATM cells into a packet and a packet routing section having a routing function.
FIG. 11 is a conceptual diagram showing an example of the above source routing method, wherein there are provided a transmission source node 11a, an ATM router 11b and a transfer destination node 11c. Herein, the transmission source node 11a transmits packets while designating a node that the packets pass through, so that the ATM router (or ATM node) 11b is designated by the transmission source node 11a in such a way that the transmitted packets pass through the ATM router 11b. Thus, the packets are finally transferred to the transfer destination node 11c. In addition, reference symbol "11d" designates an IP packet that is transmitted from the transmission source node 11a, while "11e" designates a capsulated IP packet, which is capsulated at the transmission source node 11a using an IP header whose destination address corresponds to the ATM node 11b.
The system of FIG. 11 is configured in such a way that the transmission source node 11a transfers the IP packet to the transfer destination node 11c by way of the ATM router 11b. Herein, the IP packet 11d to be transferred is capsulated in accordance with the IP header whose destination address corresponds to address B of the ATM router 11b that the packet should pass through. Then, the IP packet is subjected to ATM cell formation, so that a stream of ATM cells are transmitted to the ATM router 11b. Such an ATM cell stream corresponding to the capsulated IP packet is transmitted to the ATM router 11b by using the destination address of the IP header, which is capsulated. When receiving the ATM cell stream, the ATM router 11b temporarily assembles the ATM cells into the packet and dissolves capsulation of the packet. In accordance with destination address C which corresponds to an original destination address of the IP packet, the IP packet is transferred to the transfer destination node 11c.
As described above, the conventional source routing method is designed as follows:
The passing node dissolves capsulation of the packet, wherein the IP header designating the passing node is eliminated so that only the packet is transferred to the transfer destination node.
For this reason, the conventional method should reassemble the cells into the packet. In other words, at all of the passing nodes that dissolve capsulation of the packets, the cells corresponding to transfer data should be reassembled into the packets. With respect to the cell stream to be transferred, transfer delay should be increased as a number of the passing nodes is increased. So, there is a problem that the conventional method cannot perform routing process at high speed. In addition, the conventional method requires provision of cell assembling mechanism at each passing node. Thus, the conventional method requires a large scale of hardware configuration.
Meanwhile, the loose source routing of IP packets is designed such that due to capsulation, the passing node is provided as a part of the IP packet. Thus, a packet length should be elongated as a number of passing nodes which are designated is increased. For this reason, if the transmission source designates an extremely large number of passing nodes, the packet length of the IP packet exceeds a maximum length, so there occurs a possibility that the transmission source cannot designate the passing nodes.